The present invention relates to an acrylic rubber composition or, more particularly, to an acrylic rubber composition having good moldability and capable of giving a vulcanizate exhibiting excellent heat and cold resistance and oil resistance to be useful as a material of rubber parts of automobiles.
In the prior art, NBR rubbers are the most conventional as a rubbery material for automobile parts but it is a trend in recent years that acrylic rubbers are widely used for the application in an increasing amount by virtue of the well balanced properties of the vulcanizates in place of NBR rubbers.
A problem in the use of acrylic rubbers, as compared with NBR rubbers, is that acrylic rubbers in general have relatively low cold resistance. Several attempts have been made with an object to improve the cold resistance of acrylic rubbers. For example, firstly, the glass transition point Tg of an acrylic rubber can be decreased by using methyl acrylate or methoxymethyl acrylate capable of giving a polymer of a relatively low glass transition point in place of ethyl acrylate giving a polymer of a relatively high glass transition point as the comonomer to be copolymerized with an plasticizer, such as polyether-based or polyester-based oils, which is compatible with the polymer of the acrylic rubber to decrease the glass transition point of the resultant polymeric composition as compared with the polymer per se. The above mentioned first method has a problem that the decrease in the glass transition point of the polymer is necessarily accompanied by a decrease in the workability such as a decrease in the consistency and adhesion of the rubber composition under milling to the roller. The second method is disadvantageous because the amount of the oil to be added is so large as to be 5 to 20 parts by weight per 100 parts by weight of the rubber and the rubber compound has a decreased Mooney viscosity and the vulcanizate thereof has decreased mechanical strengths and heat resistance. Thus, acrylic rubbers having satisfactory cold resistance have not yet been obtained from the practical standpoint.
Further, it is also known that the cold resistance of an acrylic rubber can be improved by compounding the rubber with an organopolysiloxane. For example, Japanese Patent Kokai 60-195149 proposes a diorganopolysiloxane, or which from 15 to 50% by moles of the organic groups bonded to the silicon atoms are fluorine-containing hydrocarbon groups, as the organopolysiloxane additive. Japanese Patent Kokai 60-195148 proposes a method in which from 10 to 90% by weight of an acrylic polymer is compounded with 90 to 10% by weight of a diorganopolysiloxane and 0.5 to 30% by weight of a fluorine-containing polysiloxane as a compatibility improver. Japanese Patent Kokai 60-152552 proposes use of a siloxane acrylic acid ester containing aliphatically unsaturated groups as the compatibility improver. Further, Japanese Patent Kokai 55-7814 proposes use of 5 to 50% by weight of a copolymer of an organosiloxane and an acrylic acid ester as the compatibility improver. These prior art proposals each have an object to improve the miscibility of an acrylic rubber and a silicone rubber of which the miscibility is inherently poor. Accordingly, each of these methods has a problem that the amount of the silicone rubber to be compounded with the acrylic rubber must be so large as to be 10% by weight or larger based on the acrylic rubber.
Besides, it has been proposed that the properties of acrylic rubbers such as moldability can be improved by compounding the rubber with a long-chain fatty acid such as stearic acid. This method, however, is not practical, in particular, for the acrylic rubbers having improved cold resistance as mentioned above due to the remarkable adhesion of the rubber composition under milling to the surface of the milling roller even by the use of a large amount of stearic acid.